200

0 20 40 60 80 100 120 140 corrosion time, h (d)

FIGURE 9.26 (Continued).

9.6.4 Concluding Remarks

The results presented show clearly the following:

1. There is a significant degradation of tensile ductility of Al alloys with exposure time in the corrosive environment. Controlled experiments (by removing the corroded surface layers) have shown that the observed embrit-tlement is not caused by a surface damage mechanism. It is rather related to a bulk embrittlement effect.

2. The fatigue life of corroded specimens decreases significantly as well. Yet, the fatigue crack growth behavior of the materials is not affected by existing corrosion.

3. There is a considerable buildup of hydrogen in the material with exposure time in the corrosive environment. Hydrogen is trapped at different states that are related to microstructural traps. Focusing our discussion on the exfoliation corrosion test, where the hydrogen measurements were performed, one can observe that both tensile ductility and hydrogen uptake follow similar time dependence. Figure 9.27 is an effort to link the degradation of tensile ductility to hydrogen. The energy density and hydrogen content for state T4 are plotted against exposure time in the exfoliation solution. State T4 was selected as the state associated with the highest amounts of hydrogen. The figure shows that the rapid degradation of energy density in the first 30-40 h of exposure is associated with a respective rapid buildup of the hydrogen state T4 in the material. Both energy density and hydrogen saturate to their final values at about 40-50 h of exposure. This similarity of behavior is a strong indication that indeed the observed tensile ductility degradation is caused by bulk hydrogen embrittlement mechanisms. Hydrogen states T2 and T3 also show similar behavior (rapid increase followed by saturation) to the T2 state. Controlled experiments are necessary to separate the different hydrogen states and to identify the state being responsible for the observed embrittlement.

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